1. Field of the Invention
The present invention relates to a photonic crystal multilayer substrate, and in particular, to a structure of a micro lightwave circuit using a photonic crystal, a relevant multilayered structure, and a structure for realizing interlayered optical wiring, and to manufacturing methods thereof.
2. Description of the Related Art
Recently, photonic crystals functioning with two or three dimensional periodic structures with respect to the effective refractive index in the order of optical wavelengths have become the focus of attention. The size of the existing lightwave circuits (or optical circuits) may be reduced using such a photonic crystal by three or more orders of magnitude; therefore, application to micro lightwave circuits in optical communication or the like is anticipated.
In a photonic crystal, a photonic band gap for prohibiting the transmission of lightwaves with specific wavelength can be generated. If a linear defect is introduced in a photonic crystal having such a photonic band gap, lightwaves can be completely confined in the linear defect, and additionally, this photonic crystal can be used as an optical waveguide in which light is transmitted along the linear defect.
Such a photonic crystal optical waveguide may include a sharply bent portion, thereby improving the flexibility of the design of the pattern of the relevant lightwave circuit, and decreasing the size of the lightwave circuit.
In recent tests, various optical devices such as optical waveguides and the like were formed in a photonic crystal, thereby forming micro lightwave circuits.
However, the conventional photonic crystal lightwave circuits have some problems.
First, in the concept of the conventional lightwave circuits, the circuit is formed in a single plane. Therefore, even if the flexibility of the circuit pattern can be improved by employing sharply bent portions of optical wiring in the circuit by using a photonic crystal, the possible degree of integration is considerably limited.
Therefore, similarly to the multilayered structures in electronic integrated circuits on Si substrates, multilayered structures of optical wiring in lightwave circuits have been examined so as to improve the degree of integration. However, multilayered structures of optical wiring (i.e., optical waveguides) could not be easily realized, in comparison with the case of the multilayered structures in electronic integrated circuits.
The reason for this is that in optical wiring (i.e., optical waveguides), the mechanism for confining lightwaves in the optical waveguide is not as powerful in comparison with the confinement of electric current in electric wiring. Therefore, when two optical waveguides are positioned close to each other, interference (i.e., crosstalk) is generated between them. In addition, optical confinement of lightwaves is also insufficient at sharply bent portions; thus, lightwave signals may leak at such bent portions of the optical waveguide.
Furthermore, the scale of the lightwave circuit which can be formed in a single layer obviously has a limit; therefore, multilayered structures of the lightwave circuit have been strongly required. However, no concrete multilayered structure applied to the lightwave circuit and relevant manufacturing method have yet been proposed.
In consideration of the above circumstances, an object of the present invention is to solve the above-explained problems relating to the conventional lightwave circuits and to provide a lightwave circuit by which the degree of integration can be further improved by employing a multilayered structure of wiring, similar to that in electronic integrated circuits.
Therefore, the present invention provides a photonic crystal multilayer substrate having portions, each portion having a slab-waveguide type photonic crystal structure, multilayered in the direction of the thickness of the substrate, wherein in the slab-waveguide type photonic crystal structure:
a photonic crystal layer is disposed between cladding layers;
the photonic crystal layer is made of a photonic crystal having a two or three dimensional periodically modulated structure with respect to the effective refractive index in the order of optical wavelengths; and
each cladding layer is made of a material whose effective refractive index differs from the effective refractive index of the photonic crystal layer.
Preferably, the effective refractive index of the material of each cladding layer is smaller than an effective refractive index of the photonic crystal layer. For example, each cladding layer is made of a one or two dimensional photonic crystal, so that a slab-type optical waveguide can be formed, and such slab-type optical waveguides can be multiply layered, thereby considerably improving the degree of integration of optical integrated circuits.
In the above structure, a multilayered lightwave circuit can be formed by forming optical devices, such as optical waveguides, optical coupling-splitting circuits, optical-wavelength filters, light emitting elements, light receiving elements, or the like, in the photonic crystal layers.
In the above structure, each cladding layer may have a multilayered film in which two or more kinds of materials having different effective refractive indexes are alternately layered.
In addition, each cladding layer may be made of a photonic crystal having a two or three dimensional periodically modulated structure with respect to the effective refractive index.
Preferably, the multilayered film or the photonic crystal for forming the cladding layer has high optical reflectivity for the wavelengths at which the lightwave circuit is operated.
The above structure may include a base layer on which the portions having the slab-waveguide type photonic crystal structure are formed;
optical devices formed in at least two layers among the base layer and the photonic crystal layers; and
a mechanism for transmitting and receiving an optical signal between the optical devices of each layer.
It is possible that:
an optical device is formed in one of the base layer and the photonic crystal layers, and an optical waveguide is formed in the circuit plane of the optical device; and
the mechanism for transmitting and receiving an optical signal between the optical devices is a mode converter for converting a lightwave in a manner such that a lightwave, which is transmitted along the optical waveguide, is radiated in the direction significantly perpendicular to the circuit plane, or a lightwave incident on the circuit plane in the direction significantly perpendicular to the circuit plane is transmitted along the optical waveguide.
The mode converter may have an optical resonance mechanism, formed at an end portion of the optical waveguide in the circuit plane of the optical device, by which resonance for a lightwave with a certain wavelength transmitted through the optical waveguide occurs, and the lightwave transmitted through the optical waveguide is radiated in the direction significantly perpendicular to the circuit plane.
The optical resonance mechanism may have a hole having a shape and a size by which resonance with a lightwave transmitted through the optical waveguide occurs, and a resonant portion which surrounds the hole.
It is also possible that:
an optical device is formed in one of the base layer and the photonic crystal layers, and a first optical waveguide is formed in the circuit plane of the optical device;
a second optical waveguide is formed in one of the remaining base layer and photonic crystal layers and the second optical waveguide is close to an end portion of the first optical waveguide; and
the mechanism for transmitting and receiving an optical signal between the optical devices inputs a lightwave from the first optical waveguide to the second optical waveguide, and is a mode converter for converting a lightwave in a manner such that a lightwave, which is transmitted along the first optical waveguide, leaks at the end portion of the first optical waveguide, and the leaked lightwave is input into the second optical waveguide.
In this case, the mode converter may include the end portion of the first optical waveguide and the second optical waveguide, where the end portion has a tapered shape in which the thickness gradually decreases towards a head point.
According to the mechanism for transmitting and receiving an optical signal between the optical devices, optical signals can be transmitted between different layers of the photonic crystal multilayer substrate.
The present invention also provides a method of manufacturing a photonic crystal multilayer substrate, comprising the steps of:
forming a cladding layer on a first substrate;
forming a photonic crystal layer on the cladding layer, where the photonic crystal layer is used for forming a lightwave circuit;
producing a first wafer by forming a lightwave circuit in the photonic crystal layer, where the lightwave circuit is assigned to a first layer of the photonic crystal multilayer substrate;
producing a second wafer by forming a cladding layer on a second substrate and forming a photonic crystal layer on this cladding layer;
producing a composite wafer by putting the first and second wafers together in a manner such that the photonic crystal layer of the first wafer adheres with the cladding layer of the second wafer;
removing the substrate portion of the second wafer from the composite wafer;
forming a lightwave circuit in the photonic crystal layer which is exposed after the removal of the substrate portion of the second wafer, where this lightwave circuit is assigned to a second layer of the photonic crystal multilayer substrate; and
forming a multilayered lightwave circuit in the photonic crystal layers by repeating the above steps.
According to this method, a photonic crystal multilayer substrate having a desired structure including multiple layers can be manufactured.